The invention relates to an apparatus for measuring the electric power or energy in an A-C network, and more specifically to multiplying devices operating according to the principle of pulse duration-pulse height modulation.
An arrangement for generating a pulse sequence proportional to the product of two electrical analog variables is known from German Offenlegungsschrift No. 25 19 668. That arrangement, used for measuring electric power and energy, feeds an electrical analog variable, derived from the one measurement variable to be multiplied, to a current to frequency converter which includes an integrating stage, a monostable multivibrator and a reference voltage source. The reference D-C quantity from the voltage source is superimposed on the electrical analog variable for a time predetermined by the monostable multivibrator in such a manner that it opposes this variable and leads to the discharge of a capacitor in the integrating stage. The switching time of the monostable multivibrator is determined by an electrical analog variable derived from the second measurement variable in such a manner that the switching time is inversely proportional to the magnitude of the electrical analog variable. A charging and discharging of the capacitor of the integrating stage is thus caused. The frequency of the charging and discharging of the capacitor of the integrating stage is proportional to the product of the two electrical analog variables. In this connection, it has already been proposed that an input signal be derived from the measured current by means of a shunt or a current transformer and that the input signal derived from the measurement voltage be converted directly into a D-C voltage proportional to the latter, and the phase relation between the two measurement variables be taken into consideration by one of the known circuits.
An electronic kilowatt-hour meter with a multiplying device operating according to the principle of pulse duration-pulse height modulation is disclosed in German Auslegeschrift No. 23 48 667. This device generates a current or a voltage proportional to the product of load voltage and current. Through the use of a resistance network, the device derives two currents directly from the load voltage. The first current is proportional to the load voltage, and the second current is also proportional to the load voltage, but has a polarity opposed to that of the first current and halved instantaneous values. The first current is added to the second current during one of two switching states of a switch, which is actuated by a pulse duration-pulse height modulator controlled by the load current. The mean value of the resultant current is proportional to the product of the load current and the load voltage.